/*
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Iterator;
import java.util.NoSuchElementException;

import com.google.common.annotations.GwtCompatible;

/**
 * An iterator that supports a one-element lookahead while iterating.
 * 
 * @author Mick Killianey
 * @since 2010.01.04 <b>stable</b> (imported from Google Collections Library)
 */
@GwtCompatible
public interface PeekingIterator<E> extends Iterator<E> {
    /**
     * Returns the next element in the iteration, without advancing the
     * iteration.
     * 
     * <p>
     * Calls to {@code peek()} should not change the state of the iteration,
     * except that it <i>may</i> prevent removal of the most recent element via
     * {@link #remove()}.
     * 
     * @throws NoSuchElementException
     *             if the iteration has no more elements according to
     *             {@link #hasNext()}
     */
    E peek();

    /**
     * {@inheritDoc}
     * 
     * <p>
     * The objects returned by consecutive calls to {@link #peek()} then
     * {@link #next()} are guaranteed to be equal to each other.
     */
    E next();

    /**
     * {@inheritDoc}
     * 
     * <p>
     * Implementations may or may not support removal when a call to
     * {@link #peek()} has occurred since the most recent call to
     * {@link #next()}.
     * 
     * @throws IllegalStateException
     *             if there has been a call to {@link #peek()} since the most
     *             recent call to {@link #next()} and this implementation does
     *             not support this sequence of calls (optional)
     */
    void remove();
}
